Historically, transmitted optical signals were modulated by signals that change their amplitude or intensity. This had the benefit of being a straightforward approach, where Wavelength Division Multiplexing (WDM) was possible, but had the disadvantage that the capacity was limited as only a limited number of channels could fit into the passband of an Intensity Modulation Direct Detection (IMDD) system.
Coherent detection of phase modulated optical signals offers an advantage in terms of required Optical Signal To Noise Ratio (OSNR) over directly detected intensity modulated signals of an equivalent spectral efficiency. The main ways of implementing a coherent optical receiver are homodyne, heterodyne and intradyne.
The optical homodyne receiver is theoretically most straightforward. An Optical Local Oscillator (OLO) provided by a laser is phase locked to the recovered carrier so that the optical signal is directly converted to baseband. However, problems associated with the control of the laser mean that this is extremely difficult to implement in practice.
In an optical heterodyne receiver, the OLO is offset in frequency from the received signal so that it is outside the signal bandwidth. Image frequencies generated by mixing can then be removed by optical filtering (“image rejection”). This results in a requirement for an electrical bandwidth at the first Intermediate Frequency (IF) which is at least twice that required for the homodyne receiver and which may exceed the limits of available devices.
An alternative approach is phase diversity reception with an intradyne receiver which overcomes some of the shortcomings of the homodyne and heterodyne approaches by exploiting the ability to closely match the oscillation frequency of a remote transmitter with a local oscillator, using coherent sources. In an intradyne optical receiver, the optical local oscillator has a frequency within the signal bandwidth but, unlike the homodyne approach, it is not phase locked to the original carrier thereby eliminating the need for a complex control loop around the OLO laser. This requires a much narrower bandwidth than heterodyne approach because the local oscillator frequency is within the signal bandwidth. However, a consequence of lack of phase locking is also a tendency for an uncorrectable phase slip to occur between the demodulated signals and OLO.
Image rejection is not always possible since optical filters are ineffective when the IF spectrum consists of two almost superimposed spectra. An image rejection architecture for an intradyne receiver employs parallel quadrature signal paths which are mixed with local oscillators also in quadrature and then recombined such that one set of image components is cancelled. This approach requires that the parallel signal paths are closely matched in amplitude and phase to achieve the required degree of cancellation. It is difficult to maintain this degree of match between channels over a wide frequency band if the paths are not identical to each other in terms of the components contained therein.
A coherent optical intradyne system is described in Derr et al, Journal of Light Wave Technology, Vol. 10, No. 9, September 1992. This paper describes a receiver based on Quadrature Phase Shift Keying (QPSK) with a digital realisation of synchronous demodulation including phase synchronisation. This approach could, in principle, overcome the problem of matching channels, but is limited by the availability of sufficiently fast low cost digital electronics and analogue to digital converters.
A three fibre coupler is an attractive way of combining an optical local oscillator with a received signal and leading to signals which may be converted to electrical signals for processing. This works well with binary phase shift keyed (BPSK) signals, other than for the above mentioned phase slip, but in order to generate two components in quadrature with the signals on one output of the coupler, it is necessary to subtract one of the signals on the other two outputs from the other. This results in an asymmetry between the in-phase and quadrature paths, since the in-phase path does not undergo this operation.
A coherent intradyne optical receiver is described in Yamashita S, IEEE Photonics Technology Letters, Vol 6, No 11, November 1994. This receiver illustrates in FIG. 1 thereof a three-fibre coupler (the “3×3 fiber coupler”) and describes using only two of the three outputs (“beat signals”). This approach has the limitation that it cannot detect two orthogonal phase channels, since only a single channel is produced at the intermediate frequency (IF).
It will be appreciated that in addition to the use of optical frequencies for effecting transmission of signals by phase shift keying (PSK) it is also known to employ optical properties such as polarization to effect modulation by polarization shift keying (PoISK) and indeed for both modulation forms to exist together. They are alike insofar as when demodulated in an asynchronous detector resultant electrical signals slip in phase relative to a particular datum that corresponds to a reference polarization.
It will be appreciated that an optical local oscillator which exploits coherence with an optical carrier of a transmitted signal represents a special case of transmission using two mutually coherent sources.
Thus a need exists for a coherent optical receiver which at least overcomes some of the problems of the prior art receivers. For example, an optical receiver which can accommodate uncertainties (phase or polarization slip) of the optical signals carried through to electrical signals derived therefrom and permit processing of separate, but related, channels for quadrature related signals; an optical receiver which overcomes the difficulties experienced hitherto with optical diversity reception in an economic and robust manner.